The instant invention relates generally to air cycle machines used to produce a cool air supply and more particularly to an environmental control unit integrated with a gas turbine engine to provide cabin ventilation and temperature and pressure control in a jet aircraft application. Ram air and/or bypass duct air are used to cool pressurized, compressed air bled from the engine compressor. The compressed air further cools as it expands through a turbine.
Prior art aircraft environmental control units utilize high pressure air bled from the compressor section of a turbine engine to supply working fluid to an air-cycle machine for production of pressurized cooling air to an aircraft cockpit/cabin. A basic bootstrap air-cycle machine comprises a small compressor driven by bleed air mechanically coupled to a turbine. The hot compressed air exiting the compressor is introduced to a heat exchanger where it is partially cooled by cooling air obtained from, for example, an air bypass duct used to exhaust a portion of "ram" air entering the engine. The partially cooled air is then directed to the turbine of the bootstrap cycle machine and is further cooled as it expands through the turbine prior to introduction to the aircraft cabin.
Many prior art environmental control systems employ ground air fans to introduce cooling air into the heat exchangers when the aircraft is stationary. This feature provides cooler cabin air when ram air or bypass duct air is unavailable. Combined simple/bootstrap cycle air machines incorporate a fan stage between the compressor and turbine thereby reducing the number of parts required to introduce cool air into the turbine and heat exchanger and improving cost and efficiency.
Environmental control units are often equipped with water separators to remove moisture from the air prior to introduction in to the cabin. Various air filtration systems are also employed to remove particulate matter and odors deleterious to passenger health and comfort. Digital electronic controls employed to monitor and actuate system components provide maximum efficiency of operation and passenger comfort.
The use of compressed air to drive the aforementioned air-cycle machines reduces the volume of air available for engine combustion, thereby reducing available power and fuel efficiency. Additionally, since not all cooling air produced by the environmental control units is required for cabin ventilation and cooling, some energy is always wasted. While the application of digital electronic controllers to monitor system operation and cabin demand has improved the operating efficiency of existing environmental control units, a great deal of energy is still exhausted from known in the art air-cycle turbine.